We study the superposition of primordial massive particles and compute the associated decoherence time scale in the radiation dominated Universe. We observe that for lighter primordial particles with masses up to ${10}^7\mathrm{kg}$, the corresponding decoherence timescale is significantly larger than the age of the observable Universe, demonstrating that a primordial particle would persist in a pure quantum state, with its wave function spreading freely. For heavier particles, they can still be in a quantum state while their position uncertainties are limited by the wavelength of background photons. We then discuss three observational signatures that may arise from a quantum superposition of primordial particles such as primordial black holes and other heavy dark matter candidates, namely, interference effects due to superpositions of the metric, transition lines in the gravitational wave spectrum due to gravitationally bound states indicating the existence of gravitons, and witnesses of quantum entanglement between massive particles and of the gravitational field.

• Received 3 November 2021
• Accepted 18 January 2022

DOI:https://doi.org/10.1103/PhysRevD.105.043505